High speed light emitting semiconductor methods and devices

ABSTRACT

A method including: providing a transistor structure that includes a base region of first semiconductor type between semiconductor emitter and collector regions of second semiconductor type; providing, in the base region, at least one region exhibiting quantum size effects; providing emitter, base, and collector electrodes respectively coupled with emitter, base, and collector regions; applying electrical signals, including a high frequency electrical signal component, with respect to the emitter, base, and collector electrodes to produce output spontaneous light emission from the base region, aided by the quantum size region, the output spontaneous light emission including a high frequency optical signal component representative of the high frequency electrical signal component; providing an optical cavity for the light emission in the region between the base and emitter electrodes; and scaling the lateral dimensions of the optical cavity to control the speed of light emission response to the high frequency electrical signal component.

PRIORITY CLAIMS

This is a continuation-in-part of U.S. patent application Ser. No.12/655,806, filed Jan. 7, 2010, incorporated herein by reference, which,in turn, claimed priority from three U.S. Provisional PatentApplications; namely, U.S. Provisional Application Ser. No. 61/204,560,filed Jan. 8, 2009, U.S. Provisional Application Ser. No. 61/204,602,filed Jan. 8, 2009, and U.S. Provisional Application Ser. No.61/208,422, filed Feb. 24, 2009. Priority is also claimed from U.S.Provisional Patent Application Ser. No. 61/212,951, filed Apr. 17, 2009,and from U.S. Provisional Patent Application Ser. No. 61/268,119, filedJun. 9, 2009, and both of said last mentioned U.S. Provisional PatentApplications are incorporated herein by reference.

RELATED APPLICATION

The subject matter of this application relates to subject matterdisclosed in copending U.S. patent application Ser. No. ______, filed ofeven date herewith and assigned to the same assignees as the presentApplication.

GOVERNMENT RIGHTS

This invention was made with Government support, and the Government hascertain rights in the invention.

FIELD OF THE INVENTION

This invention relates to methods and devices for producing lightemission in response to electrical signals. The invention also relatesto methods and devices for producing high frequency light emission andlaser emission from semiconductor devices with improved efficiency.

BACKGROUND OF THE INVENTION

A part of the background hereof lies in the development ofheterojunction bipolar transistors which operate as light-emittingtransistors and transistor lasers. Reference can be made for example, toU.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780, 7,535,034 and 7,693,195;U.S. Patent Application Publication Numbers US2005/0040432,US2005/0054172, US2008/0240173, US2009/0134939, and US2010/0034228; andto PCT International Patent Publication Numbers WO/2005/020287 andWO/2006/093883. Reference can also be made to the followingpublications: Light-Emitting Transistor: Light Emission From InGaP/GaAsHeterojunction Bipolar Transistors, M. Feng, N. Holonyak, Jr., and W.Hafez, Appl. Phys. Lett. 84, 151 (2004); Quantum-Well-BaseHeterojunction Bipolar Light-Emitting Transistor, M. Feng, N. Holonyak,Jr., and R. Chan, Appl. Phys. Lett. 84, 1952 (2004); Type-II GaAsSb/InPHeterojunction Bipolar Light-Emitting Transistor, M. Feng, N. Holonyak,Jr., B. Chu-Kung, G. Walter, and R. Chan, Appl. Phys. Lett. 84, 4792(2004); Laser Operation Of A Heterojunction Bipolar Light-EmittingTransistor, G. Walter, N. Holonyak, Jr., M. Feng, and R. Chan, Appl.Phys. Lett. 85, 4768 (2004); Microwave Operation And Modulation Of ATransistor Laser, R. Chan, M. Feng, N. Holonyak, Jr., and G. Walter,Appl. Phys. Lett. 86, 131114 (2005); Room Temperature Continuous WaveOperation Of A Heterojunction Bipolar Transistor Laser, M. Feng, N.Holonyak, Jr., G. Walter, and R. Chan, Appl. Phys. Lett. 87, 131103(2005); Visible Spectrum Light-Emitting Transistors, F. Dixon, R. Chan,G. Walter, N. Holonyak, Jr., M. Feng, X. B. Zhang, J. H. Ryou, and R. D.Dupuis, Appl. Phys. Lett. 88, 012108 (2006); The Transistor Laser, N.Holonyak and M Feng, Spectrum, IEEE Volume 43, Issue 2, February 2006;Signal Mixing In A Multiple Input Transistor Laser Near Threshold, M.Feng, N. Holonyak, Jr., R. Chan, A. James, and G. Walter, Appl. Phys.Lett. 88, 063509 (2006); and Collector Current Map Of Gain AndStimulated Recombination On The Base Quantum Well Transitions Of ATransistor Laser, R. Chan, N. Holonyak, Jr., A. James, and G. Walter,Appl. Phys. Lett. 88, 14508 (2006); Collector Breakdown In TheHeterojunction Bipolar Transistor Laser, G. Walter, A. James, N.Holonyak, Jr., M. Feng, and R. Chan, Appl. Phys. Lett. 88, 232105(2006); High-Speed (/spl ges/1 GHz) Electrical And Optical Adding,Mixing, And Processing Of Square-Wave Signals With A Transistor Laser,M. Feng, N. Holonyak, Jr., R. Chan, A. James, and G. Walter, PhotonicsTechnology Letters, IEEE Volume: 18 Issue: 11 (2006); Graded-BaseInGaN/GaN Heterojunction Bipolar Light-Emitting Transistors, B. F.Chu-Kung et al., Appl. Phys. Lett. 89, 082108 (2006); Carrier LifetimeAnd Modulation Bandwidth Of A Quantum Well AlGaAs/InGaP/GaAs/InGaAsTransistor Laser, M. Feng, N. Holonyak, Jr., A. James, K. Cimino, G.Walter, and R. Chan, Appl. Phys. Lett. 89, 113504 (2006); Chirp In ATransistor Laser, Franz-Keldysh Reduction of The Linewidth Enhancement,G. Walter, A. James, N. Holonyak, Jr., and M. Feng, Appl. Phys. Lett.90, 091109 (2007); Photon-Assisted Breakdown, Negative Resistance, AndSwitching In A Quantum-Well Transistor Laser, A. James, G. Walter, M.Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 90, 152109 (2007);Franz-Keldysh Photon-Assisted Voltage-Operated Switching of a TransistorLaser, A. James, N. Holonyak, M. Feng, and G. Walter, PhotonicsTechnology Letters, IEEE Volume: 19 Issue: 9 (2007); ExperimentalDetermination Of The Effective Minority Carrier Lifetime In TheOperation Of A Quantum-Well n-p-n Heterojunction Bipolar Light-EmittingTransistor Of Varying Base Quantum-Well Design And Doping; H. W. Then,M. Feng, N. Holonyak, Jr., and C. H. Wu, Appl. Phys. Lett. 91, 033505(2007); Charge Control Analysis Of Transistor Laser Operation, M. Feng,N. Holonyak, Jr., H. W. Then, and G. Walter, Appl. Phys. Lett. 91,053501 (2007); Optical Bandwidth Enhancement By Operation And ModulationOf The First Excited State Of A Transistor Laser, H. W. Then, M. Feng,and N. Holonyak, Jr., Appl. Phys. Lett. 91, 183505 (2007); Modulation OfHigh Current Gain (β>49) Light-Emitting InGaN/GaN Heterojunction BipolarTransistors, B. F. Chu-Kung, C. H. Wu, G. Walter, M. Feng, N. Holonyak,Jr., T. Chung, J.-H. Ryou, and R. D. Dupuis, Appl. Phys. Lett. 91,232114 (2007); Collector Characteristics And The Differential OpticalGain Of A Quantum-Well Transistor Laser, H. W. Then, G. Walter, M. Feng,and N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007); TransistorLaser With Emission Wavelength at 1544 nm, F. Dixon, M. Feng, N.Holonyak, Jr., Yong Huang, X. B. Zhang, J. H. Ryou, and R. D. Dupuis,Appl. Phys. Lett. 93, 021111 (2008); and Optical Bandwidth EnhancementOf Heterojunction Bipolar Transistor Laser Operation With An AuxiliaryBase Signal, H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr. Appl.Phys. Lett. 93, 163504 (2008).

Semiconductor light emitting diodes (LEDs) and lasers using direct gapIII-V materials, and electron-hole injection and recombination, haveover the years led to numerous applications in display and lightwavecommunications. While semiconductor lasers typically dominatelong-distance communication links, fast spontaneous lightwavetransmitters can be an attractive solution for short range optical datacommunications and optical interconnections as their threshold-lessoperation, high fabrication yield and reduced driver and feedbackcontrol complexity significantly reduce the overall cost, form factorand power consumption of transmitters. Coupled with a proper cavitydesign, such as a resonant cavity, spontaneous light sources emitting at980 nm have been shown to achieve external quantum efficiencies(η_(ext)) as high at 27% and an emission spectral width as narrow as 5nm (see J. J. Wierer, D. A. Kellogg, and N. Holonyak, Jr., Appl. Phys.Lett. 74, 926 (1999)). However, the fastest spontaneous light sourceshown to date (a light emitting diode) employs p-doping as high as7×10¹⁹ cm⁻³ to achieve a bandwidth of 1.7 GHz (i.e., recombinationlifetime of ˜100 ps), at the cost of a reduced internal quantumefficiency to 10% or less (see C. H. Chen, M. Hargis, J. M. Woodall, M.R. Melloch, J. S. Reynolds, E. Yablonovitch and W. Wang, Appl. Phys.Lett. 74, 3140 (1999)). In practice, higher efficiency spontaneousdevices such as LEDs or RCLEDs operate with bandwidths that are lessthan 1 GHz, restricting actual commercial application of spontaneouslight transmitters (LEDs and RCLEDs) to less than 1 Gbits/s.

It has previously been proposed that the heterojunction bipolar lightemitting transistor (HBLET), which utilizes a high-speed heterojunctionbipolar transistor (HBT) structure, could potentially function as alight source with speeds exceeding ten's of GHz (see M. Feng, N.Holonyak, Jr., and W. Hafez, Appl. Phys. Lett. 84, 151 (2004); M. Feng,N. Holonyak, Jr., and R. Chan, Appl. Phys. Lett. 84, 1952 (2004); W.Snodgrass, B. R. Wu, K. Y. Cheng, and M. Feng, IEEE Intl. ElectronDevices Meeting (IEDM), pp. 663-666 (2007)). The room temperature,continuous wave operation of a transistor laser further demonstratesthat a practical radiative recombination center (i.e., undoped quantumwell) can be incorporated in the heavily doped base region of a HBLET(see M. Feng, N. Holonyak, Jr., G. Walter, and R. Chan, Appl. Phys.Lett. 87, 131103 (2005)). Due to the short base effect of tilted chargepopulation in transistors, the effective minority carrier lifetime inthe base region of the HBLETs can be progressively reduced to sub-100 psby tailoring the doping and incorporating QW(s) (see H. W. Then, M.Feng, N. Holonyak, Jr, and C. H. Wu, “Experimental determination of theeffective minority carrier lifetime in the operation of a quantum-welln-p-n heterojunction bipolar light-emitting transistor of varying basequantum-well design and doping,” Appl. Phys. Lett., vol. 91, 033505,2007; G. Walter, C. H. Wu, H. W. Then, M. Feng, and N. Holonyak, Jr.,“4.3 GHz optical bandwidth light emitting transistor,” (submitted toAppl. Phys. Lett.), 2009, supra) In practice, despite the high intrinsicspeed of the HBT, the microwave performance of an HBLET is limited byparasitic capacitances, due to factors including extrinsic carriertransport effects and to the need to include light extraction features(such as oxide apertures) not present in traditional high speed HBTdevices.

It is among the objects of the present invention to address suchlimitations of prior devices and techniques, and to improve operation oftilted charge light-emitting devices and techniques, including threeterminal light-emitting transistors and two terminal tilted charge lightemitters.

SUMMARY OF THE INVENTION

Applicant has discovered that the lateral scaling of a heterojunctionbipolar light-emitting transistor (LET) or a tilted chargelight-emitting diode can improve both electrical and opticalcharacteristics. For example, the fast recombination dynamics of theintrinsic transistor can be harnessed by scaling down an emitteraperture to reduce lateral extrinsic “parasitic-like” RC charging. Thefast spontaneous modulation speeds, together with the high yield andreliability due to ease of fabrication and threshold-less operation ofthe LET or tilted charge light-emitting diode, offer attractivealternatives to laser sources, especially for use in short range opticaldata communications and interconnections.

In accordance with an embodiment of a first form of the invention, amethod is set forth for producing a high frequency optical signalcomponent representative of a high frequency electrical input signalcomponent, including the following steps: providing a semiconductortransistor structure that includes a base region of a firstsemiconductor type between semiconductor emitter and collector regionsof a second semiconductor type; providing, in said base region, at leastone region exhibiting quantum size effects; providing emitter, base, andcollector electrodes respectively coupled with said emitter, base, andcollector regions; applying electrical signals, including said highfrequency electrical signal component, with respect to said emitter,base, and collector electrodes to produce output spontaneous lightemission from said base region, aided by said quantum size region, saidoutput spontaneous light emission including said high frequency opticalsignal component representative of said high frequency electrical signalcomponent; providing an optical window or cavity for said light emissionin the region between said base and emitter electrodes; and scaling thelateral dimensions of said optical window or cavity to control the speedof light emission response to said high frequency electrical signalcomponent.

In an embodiment of the first form of the invention, the method furthercomprises providing an aperture disposed over said emitter region, andsaid scaling of the lateral dimensions includes scaling the dimensionsof said aperture. In one version of this embodiment, the aperture isgenerally circular and is scaled to preferably about 10 μm or less indiameter, and more preferably about 5 μm or less in diameter. In anotherversion of this embodiment, the window or cavity is substantiallyrectangular, and said scaling of lateral dimensions comprises providingthe window or cavity with linear dimensions of preferably about 10 μm orless, and more preferably about 5 μm or less in diameter. In thepractice of an embodiment of the method, the high frequency electricalsignal component has a frequency of at least about 2 GHz.

In accordance with an embodiment of a further form of the invention, amethod is set forth for producing a high frequency optical signalcomponent representative of a high frequency electrical signalcomponent, including the following steps: providing a layeredsemiconductor structure including a semiconductor drain regioncomprising at least one drain layer, a semiconductor base regiondisposed on said drain region and including at least one base layer, anda semiconductor emitter region disposed on a portion of said base regionand comprising an emitter mesa that includes at least one emitter layer;providing, in said base region, at least one region exhibiting quantumsize effects; providing a base/drain electrode having a first portion onan exposed surface of said base region and a further portion coupledwith said drain region, and providing an emitter electrode on thesurface of said emitter region; applying signals with respect to saidbase/drain and emitter electrodes to produce light emission from saidbase region; providing an optical window or cavity for said lightemission in the region between said first portion of the base/drainelectrode and said emitter electrode; and scaling the lateral dimensionsof said optical window or cavity to control the speed of light emissionresponse to said high frequency electrical signal component.

In an embodiment of the further form of the invention, said emitter mesahas a substantially rectilinear surface portion, and said step ofproviding said electrodes comprises providing said emitter electrodealong one side of said surface portion of the emitter mesa and providingthe first portion of said base/drain electrode on a portion of the baseregion surface adjacent the opposite side of said emitter mesa surfaceportion. In this embodiment, the step of providing said electrodesfurther comprises providing said emitter electrode and the first portionof said base/drain electrode as opposing linear conductive strips, andsaid scaling of lateral dimensions comprises providing said window orcavity with linear dimensions of preferably about 10 μm or less, andmore preferably about 5 μm or less.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-section of a device in which embodiments ofthe improvements of the invention can be employed.

FIG. 2 is a top photographic view showing an example of the layout ofconductive contacts or electrodes of the FIG. 1 device.

FIG. 3 shows, in graph (a), the collector I-V characteristics and, ingraphs (b), the optical output characteristics, of the FIG. 1 device.The light emission is measured from the bottom of the device with alarge-area photodetector.

FIG. 4 shows, in graphs (a) and (b), respectively, the optical responseof the common-collector HBLET device to BC and EC rf input at biasesI_(B)=2 mA and V_(BC)˜0 V (condition for reverse-biased BC junction).

FIG. 5 is a plot showing F_(3dB) (in GHz) as a function of I_(B) for ECinput port modulation of the HBLET with D_(A)˜6 μm and V_(BC) at 0volts. The inset shows the optical output (detector output inmicrowatts) as a function of I_(s).

FIG. 6, plots (a) and (b), show the HBLET collector IV characteristicsfor examples with emitter sizes of (a) D_(A)=5 um and (b) D_(A)=13 μm.

FIG. 7 shows HBLET optical light output (measured from the bottom) as afunction base current, I_(B), with V_(BC)=0 V, for the three devices ofthis example, with D_(A)=5 μm, D_(A)=8 μm, and D_(A)=13 μm. The insetshows the optical spectrum as arbitrary units as a function ofwavelength.

FIG. 8 is a plot of normalized response as a function of frequency, withV_(BC)=0, for the three devices of this example with D_(A)=5 μm, D_(A)=8μm, and D_(A)=13 μm.

FIG. 9 is a plot of optical bandwidth as a function of base current forthe three devices of this example with D_(A)=5 μm, D_(A)=8 μm, andD_(A)=13 μm.

FIG. 10 is a simplified cross-sectional diagram of a tilted-chargelight-emitting diode, in which an embodiment of the invention can beemployed.

FIG. 11 is a top photographic view of the FIG. 10 device.

DETAILED DESCRIPTION

For an example of an embodiment of the invention, the epitaxial layersof the crystals used for a heterojunction bipolar light emittingtransistor (HBLET), fabricated using MOCVD, included a 3000 Å n-typeheavily doped GaAs buffer layer, followed by a 500 Å n-typeAl_(0.30)Ga_(0.60)As layer, a graded Al_(0.30)Ga_(0.70)As toAl_(0.90)Ga_(0.10)As oxide buffer layer, a 600 Å n-typeAl_(0.98)Ga_(0.02)As oxidizable layer, and then a gradedAl_(0.90)Ga_(0.10)As to Al_(0.30)Ga_(0.70)As oxide buffer layer thatcompletes the bottom cladding layers. These layers are followed by a 557Å n-type subcollector layer, a 120 Å In_(0.49)Ga_(0.51)P etch stoplayer, a 2871 Å undoped GaAs collector layer, and a 1358 Å averagep-doped 3×10¹⁹ cm⁻³ AlGaAs/GaAs graded base layer (the active layer),which includes two undoped 112 Å InGaAs quantum wells (designed for A980 nm). The epitaxial HBTL structure is completed with the growth ofthe upper cladding layers, which include a 511 Å n-typeIn_(0.49)Ga_(0.51)P wide-gap emitter layer, a gradedAl_(0.30)Ga_(0.70)As to Al_(0.90)Ga_(0.10)As oxide buffer layer, a 600 Ån-type Al_(0.98)Ga_(0.02)As oxidizable layer, and a gradedAl_(0.90)Ga_(0.10)As to Al_(0.30)Ga_(0.70)As oxide buffer layer and a500 Å n-type Al_(0.30)Ga_(0.70)As layer. Finally, the HBLET structure iscapped with a 2000 Å heavily doped n-type GaAs contact layer. Aftervarious standard etching and contact metallization steps, the completeddevices of the first example hereof have an oxide aperture diameter,D_(A), of 6 μm on 10 μm emitter mesas.

A simplified schematic of the device cross section and its top viewlayout are shown in FIGS. 1 and 2. An n+ GaAs subcollector region 105has an n-type GaAs collector region 110 deposited thereon, followed byp+ AlGaAs/GaAs base region 120, having one or more undoped InGaAsquantum wells (QWs). An emitter mesa is formed over the base, andincludes, n-type InGaP emitter layer 130, and n-type AlGaAs aperturelayer 140, and an n+ GaAs cladding layer 150. Lateral oxidation can beused to form the central aperture. The collector contact metallizationis shown at 107, the base contact metallization is shown at 122, and theemitter metallization is shown at 152. FIG. 2 shows a plan view of theFIG. 1 metallizations; that is, opposing collector contacts (commonconnection not shown), the base contact 122 including an outer annularring, and the emitter contact 152 including the inner annular ring. FIG.2 shows collector (C), base (B), and emitter (E) metallizations, and arepresentation of light emission (hν) through the aperture.

The collector I-V and optical output characteristics are shown in FIGS.3( a) and 3(b), respectively. The device exhibits a current gain β(=ΔI_(C)/ΔI_(B)) as high as 30 (or 30 dB), e.g., at I_(B)=2 mA andV_(CE)=2 V. The light emission in FIG. 3( b) is measured from the bottomof the device with a large-area photodetector. A light extractionefficiency of a single escape cone from the GaAs-air surface, assumingFresnel reflection losses for normal incidence, is approximately 1.4%.(see M. G. Craford, High Brightness Light Emitting Diodes,Semiconductors and Semimetals, Vol. 48, Academic Press, San Diego,Calif., p. 56 (1997)). The broad spectral characteristics of the opticaloutput (see inset of graph (b); FWHM=76 nm) is indicative of the widthof the spontaneous recombination of the HBLET operation. The HBLET ofthis example does not incorporate a resonant cavity, it being understoodthat the use of a resonant cavity will substantially increase opticaloutput extraction.

Operating the common-collector HBLET with the BC port as the rf-inputallows for simultaneous electrical-to-optical output conversion, andelectrical output gain at the EC output port. Due to its three-portnature, its optical output can also respond to input modulation signalsat the EC-port, although in this configuration, the device does notprovide a simultaneous electrical output gain at the BC-port. Deployingthe EC-port as the rf-input has the advantage of better matched inputimpedance (50Ω standard) for maximal power transfer. The BC-port inputimpedance is generally higher than the EC-input impedance due to thereverse-biased BC junction, and can be advantageous where high inputimpedances are desirable for maximizing circuit performances.

In an example hereof, the optical response is measured with a high-speedp-i-n photodetector with bandwidth ≧12 GHz and a 50-GHz electricalspectrum analyzer. A frequency generator (0.05-20 GHz) is used for theinput signal to the device. The optical response of the common-collectorHBLET to BC and EC rf-input modulation at biases I_(B)=2 mA and V_(BC)˜0V (condition for reverse-biased BC junction) are shown in FIGS. 4( b)and 4(a), respectively. In both cases the response bandwidth at −3 dB,f_(3 dB), is 4.3 GHz. In FIG. 5, f_(3 dB) is seen to improve from 2.8 to4.3 GHz as I_(B) is increased from 1 to 2 mA. The optical output andresponse bandwidth are shown up to I_(B)=2 mA where the optical output(see plot of inset) begins to degrade due to saturation and heating.

The optical response, H(f) may be expressed as

$\begin{matrix}{{{H(f)} = \frac{A_{o}}{1 + {j\frac{f}{f_{3\; {dB}}}}}},} & (1)\end{matrix}$

where A_(o) is the electrical-to-optical conversion efficiency, andf_(3 dB) is the bandwidth at −3 dB. f_(3 dB) is related to an effectivebase carrier recombination lifetime τ_(B) (absent stimulatedrecombination but including the effects of undesirable parasiticRC-charging time) by the relation,

$\begin{matrix}{f_{3\; {db}} = {\frac{1}{2{\pi\tau}_{B}}.}} & (2)\end{matrix}$

A value for f_(3 dB) of 4.3 GHz therefore corresponds to a τ_(B) of 37ps. Sub-100 ps recombination speeds are not readily achieved in a doubleheterojunction (DH) p-i-n light emitting diode, because equal numberdensities of electrons (n cm⁻³) and holes (p cm⁻³) are injected into theneutral undoped active region to preserve charge neutrality; therefore,an extremely high injection level and equivalently, a high chargepopulation (since I_(inject)/q=B_(rad)·n·p·Vol=n·Vol/τ_(B)) are requiredin order to achieve high recombination speeds. In a HBLET, the holes arebuilt-in by p-doping in the base, and re-supplied by an ohmic basecurrent, while the (minority carrier) electrons are injected from theheterojunction emitter. Moreover, as opposed to the charge ‘pile-up’condition in a double heterojunction p-i-n diode, the dynamic ‘tilted’charge flow condition is maintained in the base of the transistor withthe electrical collector (reverse-biased BC junction) in competitionwith base recombination. Because of the ‘tilted’ base population,current flow is a function of the slope in the charge distribution, andhigh current densities are possible without requiring extreme carrierdensities. The heterojunction bipolar transistor (HBT) n-p-n structure,therefore, possesses intrinsic advantages (in how charge is handled)over the double heterojunction p-i-n structure.

Thus, the 37 ps carrier lifetime observed in the HBLET hereof indicatesthat spontaneous recombination can be “fast”, and higher modulationspeeds are possible by further reducing the undesirable parasitics. Inaddition, due to the absence of the relaxation oscillations typicallyobserved in laser devices, and the lesser signal attenuation slope of−20 dB per decade beyond the 3 dB bandwidth in contrast to the −40 dBper decade slope of laser response, an HBLET can potentially be deployedat data rates much higher than 4.3 Gb/s, with attendant advantage forshort range optical data communications.

In further examples hereof, devices are fabricated as previouslydescribed, but with emitter aperture widths of 5 μm, 8 μm, and 13 μm,achieved by selective lateral oxidation of the n-Al_(0.98)Ga_(0.02)Aslayer (aperture layer 140 of FIG. 1). The collector I-V characteristicsfor HBLETs with aperture widths of 5 μm (plot (a)) and 13 μm (plot (b))and with V_(BC)=0 (that is, base and collector shorted) are shown inFIG. 6. FIG. 7 shows the corresponding optical light outputcharacteristic L−I_(B) as measured from the bottom-side of each of thethree devices. At comparable base currents I_(B), the device with a 5 μmaperture achieves 2.4 times higher current gain than the 13 μm device.The 13 μm HBLET, however, produces an optical output 2.4 times higher.The current gain, 13, and optical output saturate at high biasconditions (V_(CE)≧2 V) due to excessive heating as the devices are onsemi-insulating substrate and operated without any temperature control.While total recombination radiation increases for the larger device,only a fraction of the radiative recombination occurs within theintrinsic transistor base region. Due to the ‘ring’-like geometryemployed in these examples, the proper intrinsic transistor base spans aconcentric region with a radius proportional to D_(A)/2, and anintrinsic device width (active edge) denoted by, say, t. Hence, theproportion of intrinsic base recombination to the total (extrinsic andintrinsic) recombination is roughly inversely proportional to theaperture width D_(A), and hence, scales by the simple ratio,˜πD_(A)t/π(D_(A)/2)²=4t/D_(A). As the device aperture size, D_(A), isreduced, an increasingly larger proportion of the injected carriers areconfined to the intrinsic transistor base region (i.e., higher4t/D_(A)), resulting in higher current densities and enhanced currentgains. However, with a larger lateral geometry (i.e., larger D_(A) and,hence lower 4t/D_(A)), the carrier contribution to extrinsic base(radiative and non-radiative) recombination increases, resulting in alower 13 and commensurately higher light output. A typical opticalspectrum of the devices (inset of FIG. 7) shows a FWHM of 76 nm anddemonstrates that the device is operating in spontaneous recombination.The light extraction of a single escape cone from the GaAs-air surfacefor these examples is highly inefficient. Assuming Fresnel reflectionlosses for normal incidence, the extraction efficiency is estimated tobe 1.4% (see W. Snodgrass, B. R. Wu, K. Y. Cheng, and M. Feng, IEEEIntl. Electron Devices Meeting (IEDM), pp. 663-666 (2007).

In FIG. 8 the HBLET is operated in the common-collector configurationwith rf-input applied at the EC-port with V_(BC)=0 V. Although in thisconfiguration the device does not provide a simultaneous outputelectrical gain, the EC-input impedance, Z_(EC), is well matched to thesource impedance (50 standard) for maximal power transfer. In thisexample the optical response is again measured with a 12 GHz p-i-nphotodetector and a 50-GHz electrical spectrum analyzer. Also, afrequency sweep generator up to 20 GHz is again used for the inputsignal to the device. FIG. 8 shows the maximum bandwidth opticalresponse of 4.3, 2.8, and 1.8 GHz achieved by HBLETs of aperture sizeD_(A)=5, 8, and 13 μm, respectively. Higher bandwidths are attained withHBLETs employing a smaller aperture because a larger proportion ofradiative recombination is confined to the intrinsic base of the HBLETwhere the intrinsic recombination speed of the carriers are faster,consistent with the observations derived from the collector I-Vcharacteristics (FIG. 6) and optical L−I_(B) characteristics (FIG. 7).The plot of the optical bandwidth vs. the bias base current I_(B) forHBLETs of various aperture sizes (FIG. 9) shows the increase in theoptical bandwidth as the bias current (I_(B) and hence, I_(E)) isincreased. The maximum bandwidth is achieved where the optical andelectrical characteristics begin to saturate due to heating, as isevident from FIGS. 6 and 7.

In the absence of stimulated recombination, one can simply express theoptical response as a single-pole transfer function H(f) with f_(3dB)representing the −3 dB frequency. The value f_(3dB) is related to anextrinsic base carrier recombination lifetime τ_(B) byf_(3dB)=1/(2πτ_(B)). Therefore, an extrinsic τ_(B) of 37 ps is inferredfrom the value f_(3dB)=4.3 GHz (for the device where D_(A)=5 μm), whilea τ_(B) of 88 ps is obtained for a 13-μm-aperture device. Lateralextrinsic recombination therefore forms an equivalent parasitic-likeRC-charging time that limits the optical bandwidth of the device.Therefore, by lateral scaling, the device's performance can be improvedby ‘channeling’ (via high current densities) and ‘limiting’ (via smallerapertures) the carriers to feed only radiative recombination originatingor emanating from the intrinsic transistor base. Due to the presence ofa finite (parasitic) lateral edge in the device construction, the τ_(B)obtained of 37 ps is still dominated or limited extrinsically. Thisshows that the intrinsic transistor base recombination lifetime can bemuch faster than 37 ps, and implies that an even higher spontaneousoptical bandwidth is possible.

In copending U.S. patent application Ser. No. ______, filed of even dateherewith and assigned to the same assignees are the present Application,there is disclosed an embodiment of a two terminal tilted-charge lightemitting diode having a non-circular (e.g. rectangular) region as itsoptical window or cavity, between linear emitter and base electrodes orcontacts which can be opposing conductive strips. This configuration hasthe advantage of enhanced uniformity of carrier injection in the activeregion and efficient light output. The above-described scalingadvantages are also applicable to this configuration. Reference can bemade to the simplified cross-section of FIG. 10 hereof in which an n+GaAs subdrain 1005 has an n-type drain region 1010 deposited thereon,followed by p+ AlGaAs/GaAs base region 1020, having one or more InGaAsquantum wells (QWs). An emitter mesa is formed over the base andincludes an n-type InGaP emitter layer 1030, and an optional n-typeAlGaAs aperture layer 1040, and an n+ GaAs cladding layer 1050. Theemitter electrode metal is shown at 1052, and base/drain electrode metalat 1060. FIG. 11 is a top photographic view of FIG. 10 device, showingemitter electrode (E) and base/drain electrode (BD), and denoting therectangular optical window or cavity (i.e., between the upper “flange”portion of the BD electrode 1060 and the E electrode 1052) with anarrow. A similar configuration, between linear base and emitterelectrodes, can also be employed in a three terminal light-emittingtransistor or laser transistor. The above-described scaling advantagesare also applicable to these device configurations.

1. A method for producing a high frequency optical signal componentrepresentative of a high frequency electrical input signal component,comprising the steps of: providing a semiconductor transistor structurethat includes a base region of a first semiconductor type betweensemiconductor emitter and collector regions of a second semiconductortype; providing, in said base region, at least one region exhibitingquantum size effects; providing emitter, base, and collector electrodesrespectively coupled with said emitter, base, and collector regions;applying electrical signals, including said high frequency electricalsignal component, with respect to said emitter, base, and collectorelectrodes to produce output spontaneous light emission from said baseregion, aided by said quantum size region, said output spontaneous lightemission including said high frequency optical signal componentrepresentative of said high frequency electrical signal component;providing an optical cavity for said light emission in the regionbetween said base and emitter electrodes; and scaling the lateraldimensions of said optical cavity to control the speed of light emissionresponse to said high frequency electrical signal component.
 2. Themethod as defined by claim 1, further comprising providing an aperturedisposed over said emitter region, and wherein said scaling of thelateral dimensions includes scaling the dimensions of said aperture. 3.The method as defined by claim 2, wherein said aperture is generallycircular and is scaled to about 10 μm or less in diameter.
 4. The methodas defined by claim 2, wherein said aperture is generally circular andis scaled to about 5 μm or less in diameter.
 5. The method as defined byclaim 1, wherein said cavity is substantially rectangular, and whereinsaid scaling of lateral dimensions comprises providing said cavity withlinear dimensions of about 10 μm or less.
 6. The method as defined byclaim 1, wherein said cavity is substantially rectangular, and whereinsaid scaling of lateral dimensions comprises providing said cavity withlinear dimensions of about 5 μm or less in diameter.
 7. The method asdefined by claim 4, wherein said high frequency electrical signalcomponent has a frequency of at least about 2 GHz.
 8. The method asdefined by claim 6, wherein said high frequency electrical signalcomponent has a frequency of at least about 2 GHz.
 9. The method asdefined by claim 1, wherein said scaling of dimensions includesincreasing the collector region thickness to at least about 250 nm. 10.The method as defined by claim 1, wherein said step of applyingelectrical signals includes operating said semiconductor transistor in acommon collector configuration.
 11. The method as defined by claim 1,further comprising providing at least one reflector to enhanceextraction of said output spontaneous optical emission.
 12. The methodas defined by claim 1, further comprising providing an optical resonantcavity enclosing at least part of said base region, and wherein saidoutput emission comprises laser emission.
 13. A method for producing ahigh frequency optical signal component representative of a highfrequency electrical signal component, comprising the steps of:providing a layered semiconductor structure including a semiconductordrain region comprising at least one drain layer, a semiconductor baseregion disposed on said drain region and including at least one baselayer, and a semiconductor emitter region disposed on a portion of saidbase region and comprising an emitter mesa that includes at least oneemitter layer; providing, in said base region, at least one regionexhibiting quantum size effects; providing a base/drain electrode havinga first portion on an exposed surface of said base region and a furtherportion coupled with said drain region, and providing an emitterelectrode on the surface of said emitter region; applying signals withrespect to said base/drain and emitter electrodes to produce lightemission from said base region; providing an optical cavity for saidlight emission in the region between said first portion of thebase/drain electrode and said emitter electrode; and scaling the lateraldimensions of said optical cavity to control the speed of light emissionresponse to said high frequency electrical signal component.
 14. Themethod as defined by claim 13, wherein said emitter mesa has asubstantially rectilinear surface portion, and wherein said step ofproviding said electrodes comprises providing said emitter electrodealong one side of said surface portion of the emitter mesa and providingthe first portion of said base/drain electrode on a portion of the baseregion surface adjacent the opposite side of said emitter mesa surfaceportion.
 15. The method as defined by claim 14, wherein said step ofproviding said electrodes further comprises providing said emitterelectrode and the first portion of said base/drain electrode as opposinglinear conductive strips.
 16. The method as defined by claim 15, whereinsaid cavity is substantially rectangular, and wherein said scaling oflateral dimensions comprises providing said cavity with lineardimensions of about 10 μm or less.
 17. The method as defined by claim15, wherein said cavity is substantially rectangular, and wherein saidscaling of lateral dimensions comprises providing said cavity withlinear dimensions of about 5 μm or less.
 18. The method as defined byclaim 17, wherein said high frequency electrical signal component has afrequency of at least about 2 GHz. 19-22. (canceled)
 23. A device forproducing a high frequency optical signal component representative of ahigh frequency electrical input signal component, comprising: asemiconductor transistor structure that includes a base region of afirst semiconductor type between semiconductor emitter and collectorregions of a second semiconductor type; at least one region, in saidbase region, exhibiting quantum size effects; emitter, base, andcollector electrodes respectively coupled with said emitter, base, andcollector regions; whereby, application of electrical signals, includingsaid high frequency electrical signal component, with respect to saidemitter, base, and collector electrodes, produces output spontaneouslight emission from said base region, aided by said quantum size region,said output spontaneous light emission including said high frequencyoptical signal component representative of said high frequencyelectrical signal component, and; an optical cavity for said lightemission in the region between said base and emitter electrodes; thelateral dimensions of said optical cavity being scaled to control thespeed of light emission response to said high frequency electricalsignal component.
 24. The device as defined by claim 23, furthercomprising an aperture disposed over said emitter region, and whereinsaid aperture is generally circular and is scaled to about 10 μm or lessin diameter.
 25. The device as defined by claim 23, wherein said cavityis substantially rectangular, and wherein said cavity has lineardimensions of about 10 μm or less.
 26. A device for producing a highfrequency optical signal component representative of a high frequencyelectrical signal component, comprising: a layered semiconductorstructure including a semiconductor drain region comprising at least onedrain layer, a semiconductor base region disposed on said drain regionand including at least one base layer, and a semiconductor emitterregion disposed on a portion of said base region and comprising anemitter mesa that includes at least one emitter layer; at least oneregion, in said base region, exhibiting quantum size effects; abase/drain electrode having a first portion on an exposed surface ofsaid base region and a further portion coupled with said drain region,and an emitter electrode on the surface of said emitter region; wherebysignals applied with respect to said base/drain and emitter electrodesproduces light emission from said base region; and an optical cavity forsaid light emission in the region between said first portion of thebase/drain electrode and said emitter electrode; the lateral dimensionsof said optical cavity being scaled to control the speed of lightemission response to said high frequency electrical signal component.27. The device as defined by claim 26, wherein said cavity issubstantially rectangular, and wherein said cavity is scaled to havelinear dimensions of about 10 μm or less. 28-29. (canceled)